Global energy consumption is projected to increase at least two-fold by mid-century, and this increased need will be met, at least in part, through use of renewable energy sources. Due to the intermittent nature of these resources, large-scale energy storage sources must likewise be invented, developed, and deployed in this timeframe in order for these carbon neutral technologies to be fully utilized and to aid in controlling CO2 emissions. The need for grid storage is also being driven by the evolving nature of the grid (smart grid, green grid, and the distributed nature of the grid) as well as by other technological developments, such as vehicle electrification. Technologies that have been explored for various energy storage applications include pumped hydroelectric (PHE), compressed air (CAES), batteries, flywheels, and ultracapacitors. Among the technologies that are not geographically constrained, flow batteries show promise in terms of power rating (MW), response time, capital cost, and cycle life at 80 percent depth of discharge.
Broadly defined, a flow battery is an energy storage technology that utilizes reduction-oxidation (redox) states of various species for charge and discharge purposes. During the charge of a redox flow battery (RFB), electro-active material is pumped from an external reservoir into an electrochemical cell. Charge is stored in the form of chemical energy through changes in the charge state of the active material. Discharge occurs by reversing the process. Flow batteries are unique among charge storage devices because some designs can completely decouple power and energy.
The earliest flow battery designed was an iron-chromium battery. This battery contains aqueous chromium and iron solutions for the cathode and anode, respectively, and it has an open circuit potential of 1.2 V. Despite the low cost of the materials, this battery displays significant crossover of the electro-active species and thus significantly reduced capacity. In addition, the chromium redox reactions are sluggish and require a catalyst for reasonable performance. In order to mitigate crossover issues, an all-vanadium battery was developed with aqueous vanadium solutions for both the cathode and the anode. In the cathode, the vanadium cycles between the +5 and +4 oxidation states, and in the anode it cycles between +3 and +2. Like the iron-chromium chemistry, the all-vanadium battery has very low energy density due to the limited solubility of the electro-active material. In addition, the cathode displays significant temperature sensitivity that requires extensive thermal management. A promising aqueous flow battery in terms of energy density is the zinc-bromine system. However, a number of issues are still present with this chemistry, including bromine toxicity, zinc dendrite formation, and high self-discharge.
Recently a number of non-aqueous flow battery chemistries have emerged. Non-aqueous systems are of particular interest because they can have voltages that extend well beyond the electrolysis limit of water (˜1.5 V). The earliest non-aqueous chemistry developed utilized ruthenium and iron bipyridine complexes. More recently, a series of metal acetylacetonate complexes have been developed. In all of these systems the same metal complex (in different oxidation states) is used as both the cathode and anode. A key problem is the low solubility of the electro-active species in organic solvents due to their generally lower dielectric constants. One way this has been avoided is to use semi-solid slurries of common lithium ion battery materials.